1. Field of the Invention
The present invention relates to a frame holding apparatus for use in wire bonders and more particularly to a bonding level adjustment mechanism in wire bonders.
2. Prior Art
In wire bonders, a wire passes through a bonding tool which is at the end of bonding arm. The bonding tool is moved in the X and Y horizontal directions and in the Z vertical direction so that the wire is connected to a first bonding point and a second bonding point of a workpiece.
Structures for moving the bonding tool in a vertical direction include a bonding arm which is rotated about its supporting shaft without any vertical movement of the supporting shaft. This technique is disclosed in the Japanese Patent Application Publication ("Kokoku") No. 63-52778. In this prior art, the bonding tool is raised and lowered when the bonding arm is rotated. Accordingly, depending upon the height of the bonding point level (hereafter called "bonding level"), the bonding tool is caused to tilt, thus causing some problems.
Generally, at the first bonding point, a ball is formed at the end of the wire that passes through the bonding tool and the bonding is performed using the ball. Thus, since there is a ball at the tip of the bonding tool, the ball can more or less absorb the bonding level discrepancies, if any. As a result, on the first bonding point side, a difference caused by the discrepancies will not create serious problems, and favorable bonding is accomplished.
However, at the second bonding point, only the wire is present; in other words, there is no ball on the second bonding point. Accordingly, when bonding is performed by pressing the wire 3 with the bonding tool 2, the axial line 6 of the bonding tool 2 can incline with respect to a predetermined perpendicular line 5 depending on the height of the bonding level 4 as shown in FIGS. 8(a) and 8(b). FIG. 8(a) and FIG. 7(a) show the bonding level 4 too high and therefore the bonding tool 2 is inclined to the right. FIG. 8(b) and FIG. 7(b) illustrate the bonding level 4 too low and therefore the bonding tool 2 is inclined to the left.
If the bonding level 4 is thus out of adjustment (too high or too low), the thickness dimension T of the press-bonded wire 3 cannot be consistent, and this causes a considerable variation in the strength of bonding. Furthermore, when the wire 3 is pulled and cut off after the bonding to the second bonding point is completed, such a cutting of the wire tends to occur at the weakest point of the wire where the T dimension of the wire is the smallest. This causes variations in the size of the press-bonded area. Accordingly, it is necessary to minimize the variation in the T dimension in order to obtain the best possible bonding result.
Adjustment of the height of the bonding level 4 can be accomplished by controlling the upward movement limit position of the heater block that supports the workpiece. One example of a frame holding apparatus with an adjustable upper limit position of the heater block is described in the Japanese Utility Model Application Publication (Kokoku) No. 2-9556.
In this prior art, a rod which is fastened to the undersurface of the heater block is moved up and down by a spring which urges the rod upward. The up and down motions of the rod is accomplished by a heater block raising-and-lowering cam which is set as a lift (the amount of up and down motions) which makes it possible to move the heater block by a predetermined extra stroke. The upper limit position of the heater block is controlled by an engagement part of the rod which comes into contact with a stopper that is screwed to a fixed part of the apparatus.
Thus, in this prior art, the bonding level is altered by changing the position of the stopper. Accordingly, there is no standard guide for the bonding level and also there is no reproducibility. In addition, the adjustment work requires special skill and a considerable amount of time.